In recent years, technologies which realize ultrahigh-grade vacuums, and technologies which create ultraclean low pressure atmospheres by means of the inflow of small amounts of specified gasses into a vacuum chamber, have become extremely important.
Such technologies are widely used in research into the characteristics of materials, the formation of various types of thin films, and the production of semiconductor devices, and therefore higher and higher degrees of vacuum are being realized; however, furthermore, the realization of a low pressure atmosphere in which contamination by impurity elements or impurity molecules is limited to an extreme degree has been greatly desired.
For example, to use the example of semiconductor devices, as a result of the increase in the degree of integration of integrated circuits, the dimensions of the unit elements have become smaller each year, and as semiconductor devices having dimensions such that the spaces between elements have gone from a level of 1 micrometer to a submicrometer level, and further to a level less than 0.5 micrometers, have come into common use, research and development in this area has been conducted on a large scale.
The production of this type of semiconductor device is accomplished by means of the repetition of a process in which a thin film is formed, and a process in which this thin film is subjected to etching in a specified circuit pattern. It is common for this type of process to be conducted in an ultrahigh vacuum state or in a low pressure atmosphere in which a specified gas is introduced, by means of placing a silicon wafer in a vacuum chamber. In such processes, if contamination with impurities is present, for example, problems will be caused in that the quality of the thin film will be reduced, and sufficient accuracy will not be obtainable in the very detailed treating. This the reason why an ultrahigh vacuum and an ultraclean low pressure atmosphere have been desired.
One of the greatest obstacles to the realization of an ultrahigh vacuum or an ultraclean low pressure atmosphere has, up until now, been gas which was discharged from the stainless steel surface which is widely used in the chamber and in the gas pipes. In particular, it has been determined that the greatest source of contamination is from moisture which adsorbs to the surface and desorbs in a vacuum or in a low pressure atmosphere.
FIG. 5 is a graph showing the relationship between gas contamination and the total leak amount (the sum of the discharge gas amount from the surfaces of the pipe system and the interior of the reaction chamber and external leaks) of a system in a conventional device in which a gas pipe system and a reaction chamber are combined. The plurality of lines in the drawing indicate cases in which the flow amount of the gas is changed to various values as a parameter.
Semiconductor processing is exhibiting a tendency to reduce the gas flow amounts to a greater and greater extent in order to realize highly accurate processing; for example, it has now become common to use flow amounts of 10 cc/min or less.
Assuming a flow amount of 10 cc/min, if, as in presently widely used devices, a system total leakage on the order of 10.sup.-3-10.sup.-6 Torr.l/sec is present, the gas purity will be 10 ppm-1%, which is well outside highly clean processing ranges.
The present inventors have invented a ultrahigh-purity gas supply system which has succeeded in reducing the leakage amount from the exterior of the system to a level of less than 1.times.10.sup.-11 Torr.l/sec, which is below the detecting threshold of present detectors.
However, as a result of leaks from the interior of the system, that is to say, as a result of gas components discharged from the above-described stainless steel surfaces, it has been impossible to reduce the impurity concentration of the low pressure atmosphere. The minimum value of the surface discharged gas amount obtained by means of the surface treating available in the present ultrahigh vacuum technology is 1.times.10.sup.-11 Torr.l/sec.cm.sup.2 in the case of stainless steel, and even if the surface area which is exposed in the interior of the chamber is estimated at a value which is as small as possible, for example, 1 m.sup.2, a total leakage amount of 1.times.10.sup.-7 Torr.l/sec results, and a gas having a purity of only approximately 1 part per million with respect to a gas flow amount of 10 cc/min can be obtained. If the gas flow amount is further reduced, it is of course obvious that the purity will further decline.
In order to reduce the degassing component from the inner surfaces of the chamber to a level of approximately 1.times.10.sup.-11 Torr.l/sec, which is equal to the external leakage amount of the total system, it is necessary to reduce degassing from the stainless steel surfaces to less than 1.times.10.sup.-15 Torr.l/sec.cm.sup.2 ; for this reason, a stainless steel surface treating technique which can reduce the gas discharge amount has been greatly desired.
On the other hand, in semiconductor production processes, a great variety of gasses are in use, from relatively stable common gasses (O.sub.2, N.sub.2, Ar, H.sub.2, He), to rare gasses having great reactivity, corrosivity, and toxicity. In particular, if moisture is present in the atmosphere in a special gas, this may hydrolyze, producing hydrochloric acid or hydrofluoric acid, and boron trichloride (BCl.sub.3) and boron trifluoride (BF.sub.3) and the like, which exhibit strong corrosivity, will be present. Normally, stainless steel is used as a material for pipes and chambers handling such gasses, in view of its reactivity, resistance to corrosivity, high strength, ease of secondary working, ease of welding, and ease of polishing the inner surfaces thereof.
However, although stainless steel has superior resistance to corrosion in an atmosphere of dry gas, it corrodes easily in an atmosphere of a chlorine or fluorine system gas in which moisture is present. As a result, it is necessary to conduct corrosion resistant treating after the surface polishing of the stainless steel. Among such treatings, coating of a metal which has superior resistance to corrosion, such as Ni--W--P, onto the stainless steel is known; however, in this method, not merely are cracks and pin holes and the like easily caused, but as this is a method which uses wet plating, there are problems in that the amount of moisture adsorption or the residual solution component on the inner surface is large, and the like.
An example of another method is corrosion resistant treating by means of passivation treating which creates a thin oxide film on a metal surface. If sufficient oxidizer is present in a liquid, stainless steel can be passivated simply by means of immersion, so that normally, passivation treating is conducted by means of immersion in a nitric acid solution at normal temperatures.
However, this method is also a wet method, so that moisture and residual plating solution are present in large amounts on the pipes and on the inner surface of the chamber. In particular, in the case in which the moisture discharges chlorine system and fluorine system gasses, severe damage is caused to the stainless steel.
Accordingly, the construction of a chamber or gas pipe system by means of stainless steel having formed thereon a passivated film which does not receive damage even from corrosive gasses and which has low occlusion and adsorption of moisture is extremely important in very high vacuum technologies and in semiconductor processing; however, previously, this type of technology has not been available.
The present applicants have, on Feb. 4, 1988, filed a patent application Japanese Patent number 2459/88 for a stainless steel member, wherein the percentage of Ni atoms in an outer layer of an oxide film formed on a stainless steel member surface which was subjected to electrolytic polishing treating is less than 2%, and the percentage of Cr atoms in an inner layer thereof is more than 30%, and the thickness of this oxide film is within a range of 10-50 nm, and for a stainless steel member and production method, wherein heat treating is conducted in an atmosphere of an oxide gas having a moisture dew point of from -10.degree. C.-&lt;-105.degree. C. (Applicant: Tadahiro Ohmi)
This stainless steel member enables the simple conducting of desorption of the moisture by means of conducting appropriate baking, even if moisture adheres or is adsorbed, and this stainless steel member also has a small gas discharge amount from the member itself.
However, as the effects on the characteristics of the semiconductor processing and the like which are caused by the purity of the treating gas have become clearer, and as it has come to be understood that as the purity increases, devices with greater abilities can be obtained, the development of a stainless steel member which has an even smaller gas discharge amount, and furthermore, is able to more easily control the discharge of adsorbed gasses, has been strongly desired.